This invention relates to a short-circuit detector.
Since electric cars carry an electric power source of a very high voltage (about 400V), it has been proposed that a short-circuit detector be also provided. Japanese Patent Publication Tokkai 8-163704 disclosed a device adapted to detect a short circuit by connecting a relay junction to a terminal of each polarity of a high-voltage DC source and closing only one of them to detect a short circuit on the basis of a voltage drop in a detection resistor. As a second example, Japanese Patent 3041150 (Tokkai 7-20185) disclosed a detector having detection resistors connected in series between the terminals of a high-voltage DC source each through a protection resistor and detecting a short circuit at each terminal on the basis of the difference between the voltage drops in the detection resistors. As a third example, Japanese Patent 2838462 (Tokkai 6-153303) disclosed a device adapted to detect the leakage on the side of each terminal by operating a switch to measure the voltages generated across detection resistors on both sides of a high-voltage DC source. As a fourth example, Japanese Patent 3107944 (Tokkai 6-308185) disclosed a device having a protection resistor and a detection resistor connected to each terminal of a high-voltage DC source and a switch connected in parallel with each of the protection resistors such that a short circuit can be detected by switching on and off these switches.
If short circuits occur on the sides of both terminals simultaneously, however, detectors of the first three kinds cannot detect them. Detectors of the first example are not provided with means for measuring the source voltage and hence cannot detect a short circuit accurately because of the effect of variations in the source voltage. Detectors according to the second and fourth examples are not desirable from the point of view of safety because the high-voltage source is always in a connected condition with the body of the vehicle.
It is therefore an object of this invention to provide a safely operable short-circuit detector capable of dependably detecting a short circuit even in the presence of variations in the source voltage.
A short-circuit detector embodying this invention may be characterized as having a series connection including sequentially a first switch, a first detection resistor, a second detection resistor and a second switch, connected between a positive terminal and a negative terminal of a high-voltage DC power source, a third switch between a grounding terminal such as a vehicle chassis and a point between the first detection resistor and the second detection resistor, and a voltage detector circuit connected in parallel with the first and second detection resistors. A control circuit is further connected to the voltage detector circuit, serving to calculate a voltage value of the DC power source by switching on only the first switch and the second switch, to calculate a first voltage drop value across the first detection resistor by switching on only the first switch and the third switch, to calculate a second voltage drop value across the second detection resistor by switching on only the second switch and the third switch, and to determine presence or absence of short-circuited conditions at the positive terminal and the negative terminal of the DC power source on the basis of the calculated voltage value, the first voltage drop value and the second voltage drop value.
Correspondingly, a method of detecting a short-circuit condition embodying this invention may be characterized as comprising the steps of connecting the positive and negative terminals of a DC power source with a series connection including sequentially a first switch, a first detection resistor, a second detection resistor and a second switch, connecting a third switch between a grounding terminal such as a vehicle chassis and a point between the first detection resistor and the second detection resistor, connecting a voltage detector circuit in parallel with the first and second detection resistors, calculating a voltage value of the DC power source by switching on only the first switch and the second switch, calculating a first voltage drop value across the first detection resistor by switching on only the first switch and the third switch, calculating a second voltage drop value across the second detection resistor by switching on only the second switch and the third switch, and determining presence or absence of short-circuited conditions at the positive terminal and the negative terminal of the DC power source on the basis of the calculated voltage value, the first voltage drop value and the second voltage drop value.
With the detector and method according to this invention, the voltage value of the DC power source can be calculated by switching on the first and second switches together. Next, only the first and third switches are switched on and only the second and third switches are switched on. The leakage resistance values to the grounding terminal such as the chassis of a vehicle are calculated on the basis of the voltage drops across the second and third detection resistors.